1. Field of the Invention
This invention pertains generally to conjugated polymer thin film technology, and more particularly, to the use of polymerizable species as the ionically conductive species for the formation of p-i-n junctions in conjugated polymer thin films.
2. Description of Related Art
In the past 10 years or so, there has been a flurry of research on the use of neutral, undoped conjugated polymers as new semiconductor materials. Polymer light emitting diodes have indeed evolved into one of the most promising flat panel display technologies. Conjugated polymers are also explored for other semiconductor devices such as thin film transistors, solar cells, photodetectors, and so on. Due to limited carrier mobilities that are typically 2 to 4 orders of magnitude lower than single crystal silicon, polymer semiconductors in the foreseeable future will not replace silicon in high-speed devices but may find applications for which large area, light weight and flexibility are important.
Semiconductor devices made from silicon and other inorganic compounds are typically based on p-n or p-i-n junctions, formed by doping with hetero-atoms. In this context, the term “doping” is defined as when the polymers are oxidized (p-doped) or reduced (n-doped). The charges introduced onto the polymer chains are compensated by anions or cations (the dopants). Note that doping is sometimes used in the literature for simple admixing of a compound(s) into a polymer matrix.
Unlike silicon devices, conjugated polymers have been used in their undoped form in semiconductor devices, for several reasons. First, neutral conjugated polymers can be rendered soluble by the introduction of long alkyl side chains. These soluble polymers, however, become insoluble as soon as they are doped to have a high conductivity. Although there are techniques to disperse doped conducting polymers in water or organic solvents, high content of dispersion-enhancing agents are used. The dopants and the dispersing agents are mobile and therefore not suitable for the formation of stable p-n or p-i-n junctions. Secondly, it is difficult to carry out in-situ doping of conjugated polymer thin films in a controllable fashion (both doping depth and density). Even if such doping can be done, the dopants introduced in-situ are mobile. Finally, n-type doped conjugated polymers are usually unstable and susceptible to degradation by oxygen and water. Formation of p-n or p-i-n junctions in conjugated polymers has, therefore, been difficult to achieve. Without such junctions, the low mobilities of polymers, generally on the order of 10−1 cm2V−1s−1 or lower, become more problematic. Charge carriers may not be able to reach the charge-collection electrodes or charge recombination centers before they are trapped.
The in-situ formation of a dynamic p-i-n junction in conjugated polymers composited with a solid electrolyte has been demonstrated (U.S. Pat. No. 5,682,043 (1997), assigned to Uniax Corporation). The junctions were initially considered as p-n junctions, but were also treated as p-i-n junctions to reflect the significance of the neutral undoped region between the p- and n-doped regions. There has been no evidence to favor one over the other. It is likely that the junctions could be either p-n or p-i-n, depending on the polymer composition and device driving condition.
In the so-called polymer light emitting electrochemical cells (LECs) schematically shown in FIG. 1, a dynamic p-i-n junction is created when a voltage equal to or greater than the bandgap potential of the conjugated polymers is applied on thin films of the polymer composites. Under the external electric field, the anions of the electrolyte drift to the anodic side where the conjugated polymers are p-doped, while cations travel to the cathodic side to n-dope the polymers. An ionically conductive polymer such as polyethylene oxide can be admixed to enhance the ionic mobility. This p-i-n junction in the LECs is dynamic—as soon as the bias voltage is removed, the p- and n-doped regions will deplete each other while the oppositely charged dopants migrate together to recombine. The processes in FIGS. 2A and 2B are reversible. Under a constant bias voltage, the junction can be quite stable, as indicated by the relatively long lifetime of certain LECs, up to a few thousand hours. The n-doped polymers are generally unstable in air. The two electrodes, which function as oxygen and moisture barriers, may help prolong the lifetime of the LECs.
The dynamic p-i-n junction has several desirable benefits. First, the polymer/electrode interfaces are turned into ohmic contact due to the high conductivity of the doped regions. Therefore, there is no need to match the work function of the cathode to the polymer's LUMO and the anode to HOMO. Stable metals can be used as the electrodes. Next, one may ignore the low mobility of the polymers and use fairly thick polymer films in sandwich-structured devices. When the films are thick, the i region in the junction is only a small fraction of the whole film. This is reflected in FIGS. 3 and 4, in which the i region is about one tenth of the entire film. The majority of the film is in a doped region and is highly conductive. Finally, the built-in potential of the p-i-n junction can be close to the bandgap potential of the conjugated polymers, i.e., 1 to 3 V, depending on the chemical structure of the polymers. This built-in potential can provide a large open-circuit voltage for photovoltaic applications.
On the other hand, the dynamic p-i-n junction also has undesirable features. The formation of the p-i-n junction relies on the redistribution of dopants, which takes time. The turn-on time for the LECs varies from 0.1 second up to a few hours. An external bias is required to sustain the junction. Therefore, this dynamic junction is not useful for transistors and photovoltaic devices. Another problem is that any degradation in the doped regions will cause shifting of the p-i-n junction. It has been observed that in some LECs, the junction slowly shifts toward the cathode and eventually touches one of the contact electrodes.
It has been reported that the p-i-n junction in an LEC could be stabilized by cooling the device well below the glass transition temperature of the ion-transport polymer. The LECs with frozen p-i-n junctions exhibit characteristic behaviors of light emitting diodes (LED), including diode rectification, uni-polar light emission at the same polarity as that used for generating the junction, and fast response. In addition, as the ion motion is frozen, the polymer LECs could be driven at bias voltages beyond the electrochemical stability window. A similar frozen junction was also obtained at room temperature using crown ethers as the ion transport medium whose glass transition temperature is above room temperature. The resulting LECs exhibit high electroluminescence efficiency and brightness. However, frozen junctions still have shortcomings, such as a low rectification ratio and slow drifting of dopants.